Laser controlled spark-gap means for a circuit for providing a magnetic field



March 31, 1970 s. H. BARBINI 3,504,232-

A CIRCUIT FOR GNETIC LD LASER CONTROLLED SPARK-GAP MEANS PROVDING A MA Filed April C SPARK-GAP United States Patent O M 3,504,232 LASER CONTROLLED SPARK-GAP MEANS FOR A CIRCUIT FOR PROVIDING A MAGNETIC FIELD Spartacus H. Barbini, Chaville, France, assignor to Compagnie Generale dElectricite, Paris, France Filed Apr. 25, 1967, Ser. No. 633,489 Claims priority, application7 1France, Apr. 29, 1966,

Inf. ci. Hosb 37/00 U.S. Cl. 315-238 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to electrical circuits for supplying a winding with an extremely high current for a relatively short duration in order to produce a very intense magnetic ield.

More particularly, the present invention relates to devices sometimes called magnetic bottles, intended to create an intense magnetic lield in an enclosure where a very high temperature plasma is produced.

It is known that the obtention of very high temperature plasma is possible only if the plasma is concentrated and separated from the cold walls of an enclosure.

The concentration of the plasma along a column (pinch effect) is usually obtained by a strong magnetic field generated by an apparatus generally called a magnetic bottle.

The energy used by such devices is very significant and can attain several kilojoules, which is usually provided by the discharge of high voltage capacitors. The optimum use of the energy of the capacitors reduces losses of the magnetic bottles to a minimum.

It is heretofore known to discharge a capacitor into a winding by means of a first spark-gap having a relatively strong electrical rigidity and then to obtain a strong current passing through the winding by closing the circuit of the winding by means of a second spark-gap. This second spark-gap is of lower electrical rigidity, having a lower impedance and discharging the winding with a potential drop substantially equal to zero. The shock wave produced by the discharge of the capacitor into the winding and then the discharge of the winding in a very low impedance load produces in the winding, a high current resulting in significant concentration of electromagnetic energy in the Winding.

The above-described discharges are of the oscillatory type (-inductance of the winding and capacitance of the circuit including the capacitors and the spark-gap). If the losses are small, the voltage and the current are i1. quadrature, the maximum voltage corresponding to the minimum current and Vice-versa.

The present invention relates to a device or circuit 3,504,232 Patented Mar. 31, 1970 ICC capable of supplying a magnetic bottle with a high -current which is maximum when the voltage passes through zero for the rst time and which slowly decreases following an aperiodic law of variation.

According to the present invention, the system comprises, in series, a capacitor, a first spark-gap and a winding, and a second spark-gap connected in parallel across the winding. The spark-gaps are triggered by the' high energy luminous beam of a single laser, said sparkgaps being aligned and said beam passing through bores provided -in the electrodes defining the spark-gaps.

According to another feature of the invention, the triggering of said second spark-gap is obtained with a predetermined delay over the first spark-gap, the delay being equal to the time necessary for the laser beam t0 destroy a dielectric screen positioned between the electrodes of said second spark-gap.

Accord-ing to another feature of the invention, the dielectric screen consists of a plate made of a dielectric material metal coated on both sides.

According to a further feature, the system of the in#I vention comprises a switch, connected in parallel with the winding and comprising fast acting control means connected to an electric energy storage through a third spark-gap which is triggered by at least a part 0f the laser beam triggering the other spark-gaps.

According to another feature, an energy storage means is connected through a fourth spark-gap to a load placed in the magnetic eld of said winding, said fourth sparkgap being triggered by the same portion of the laser beam as said third spark-gap.

The present invention will now be described With reference to the attached drawings in which:

FIGURE 1 is a schematical representat-ion of an embodiment of the invention;

FIGURE 2 is a magnilied view of a spark-gap shown in FIGURE 1, and

FIGURE 3 shows several Icurves representing the variation of the current and the voltage in the circuit of the invention.

In FIG. 1, a triggered type laser lis shown at 1; the laser beam 2 is partly reflected into beam 2 by an element such as a semi-transparent mirror 3 which transmits a beam 2. A spark-gap A, having a relatively strong electric rigidity, comprises two electrodes 4 and 5 which present an axial bore for letting through the laser beam 2'. The electrode 5 defines with an electrode 6 an auxiliary spark-gap B. The gap of the spark-gap B is much narrower than that of the spark-gap A, but the electric rigidity is strengthened by a screen 7 made of a dielectric material metal coated on its both faces. The electrode 6 is connected to ground and the electrode 4 is connected to a high voltage source of significant capacity. Preferably, this source consists of one or more charged capacitors 8. The electrode 5 is connected to ground through a load, for example, a winding 9 which is the winding defining the magnetic enclosure.

A high current switch 10 is connected across the terminals of the winding 9. The switch 10 is controlled by a device 11 which can be, for example, a control coil or the like supplied with current by a line 11.

The control current of the line 11' is supplied by a capacitor 21 through a spark-gap C defined by the electrodes 22 and 23, the spark-gap C comprising also a dielectric screen 24. The electrode 22 comprises a central bore in 3 which is placed a lens 25. The laser beam 2 is split into two beams 2 and 2, respectively, and the latter can pass through the electrode 22, the lens 25 and impinge the dielectric screen 24.

FIGURE 2 is a magnified sectional view of the sparkgap B wherein 12 designates a focusing lens, 7 shows the dielectric screen whereas 7a and 7b are metal layers deposited on the faces of the screen 7. As shown in this figure, the laser beam 2' is focused on the outer surface of the screen 7.

The assembly of FIGURE l operates as follows: the laser 1 being triggered, the laser beam 2 triggers an arc between the electrodes 4 and 5 defining the spark-gap A. The voltage V across the capacitor 8 is then transferred to the electrode 5. The laser beam 2' impinges the screen 7 but the spark-gap B is triggered after a certain time delay because the destruction of the screen 7 is not instantaneous. The delay is a Well determined function of the nature of the screen, its thickness and of the power of the laser beam. As soon as a hole is pierced through the screen, an arc is triggered between the electrodes 5 and 6 and this arc entirely destroys the screen 5. The arc, triggered in the spark-gap B, is very strong because of the very small distance between the electrodes 5- and 6 and also because the metal layers 7a and 7b (made of a good electrical conductivity metal such as copper) are volatized at the time of the destruction of the screen 7. The thus obtained metal particles or drops form a space charge increasing the electrical conductivity of the spark-gap.

Accordingly, as soon as the spark-gap A is triggered, a damped oscillation appears in the circuit formed by the Winding 9, the capacitor 8 and also by the capacitance of the whole circuit including the spark-gap.

FIGUR-E 3 shows, as a function of the time, the variation of the voltage across the capacitor 8 (curve v), of the current supplied by the capacitor (curve i) The starting voltage being V0, the current increases While the voltage decreases and passes through zero. At this time r the spark-gap B is triggered and its resistance is very small (in the order of some l/ 100052) and the winding 9 can be discharged through this Very small load. The oscillatory discharge, illustrated by the curves in dotted lines, is then ended and the variation of the current and the. voltage follows the pattern shown in plain lines.

The time 1- can be determined by experimentation by means of an oscilloscope and can be in the order of some micro-seconds.

The triggering of the spark-gap B can be delayed with respect to the triggering of the spark-gap A by adjusting the thickness of the screen 7. Usually the thickness of the screen 7 will be in the order of some 1/10 of millimeter and the obtained delay can be reproductible with a substantially good approximation.

The installation described above enables the obtention of a monotone and relative slow decrease of the current together with a high concentration of reactive energy'in the winding 9. In order to obtain a further reduction of the losses, the conduction of the spark-gap B can be increased by means of a switch capable of transmitting significant currents. At a time T (which advantageously can be of some to 200 microseconds) the control coil 11 of the switch 10 is energized by a strong current supplied by a capacitor 21 through the spark-gap C.

The triggering of the spark-gap C is effected by the laser beam 2 which impinges the screen 24 and the time at which C is triggered determined by precise adjustment of the thickness of the screen 24 and also by an adequate selection of the material making up this screen. Usually metallization of the faces of the screen 24 proves unnecessary.

The fast closure of the switch 10 due to the sudden appearance of the current in the circuit of the control winding 11 results in the decrease of the resistance of the load connected across the winding 9 (the resistance being then equal to some microohms). The portions M and N of the curves show respectively the variation of the current and the voltage when the switch 10 is closed whereas the portions M and N show the corresponding values when the switch 10 remains open. It is, therefore, possible to obtain the maximum possible value of the magnetic field in the winding 9.

The coordination of the operation of the abovedescribed circuit with a related phenomenon (for example formation of a plasma within the magnetic field of the winding 9) can easily be achieved by using a fourth sparkgap triggered by one of the laser beams already used for triggering the other spark-gap.

This is illustrated in FIGURE 1 where 34 designates a discharge tube filled with a gas and provided with electrodes, one of which is connected to the ground, the other being connected to an energy storage means (for example a capacitor 8) through a spark-gap D defined by electrodes 32 and 33 provided with central bores letting through the laser beam 2". The discharge tube 34 is supposed to be placed in the magnetic field of the winding 9 and the triggering of the laser 1 will produce an electric discharge through the tube 1, the plasma being confined by the magnetic field of the winding 9.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A system for creating a strong magnetic field comprising an electrical energy source means connected in series with a first spark-gap means and an inductance, a second spark-gap means connected in parallel with said inductance, means including a high energy laser for triggering said first and second spark-gap means by a luminous beam, and means for delaying the triggering of said second spark-gap means comprising a dielectric screen positioned in the path of the laser beam between the electrodes of said spark-gap means.

2. The system as claimed in claim 1 wherein said dielectric screen comprises a plate of dielectric material with at least one face thereof being metal coated.

3. The system as claimed in claim 2 wherein said screen is of a thickness such that said beam pierces the same at the moment when the value of the voltage across said first spark-gap means passes through Zero.

4. The system as claimed in claim 1 further including a switch connected in parallel with said inductance.

5. The system as claimed in claim 4 further including means for operating said switch, a second electrical energy storage means, and a third spark-gap means connected between said means for operating said second electrical energy storage means, said third spark-gap means being triggered by said laser beam.

6. The system as claimed in claim 5 wherein said third spark-gap means includes two electrodes, one of which is provided with a bore for the passage of said laser beam, and a dielectric screen disposed within said third spark-gap means, in the path of said laser beam for delaying the triggering of said third spark-gap means.

7. The system as claimed in claim 6 further including means for diverting a portion of a single luminous laser beam passing through said first spark-gap means to said third spark-gap means.

8. The system as claimed in claim 1 further including: a glass tube provided with two electrodes, means for connecting one of said glass tube electrodes to the ground, third electrical energy storage means, fourth spark-gap means defined by two electrodes and provided with bores for the passage of a laser lbeam therethrough, said other glass tube electrode being connected to said third electric energy storage means through said fourth spark-gap means and said inductance comprises a winding surrounding said glass tube.

9. The system according to claim 1 wherein said first spark-gap means comprises a first and a second electrode,

said rst and second electrodes having aligned bores therethrough for the passage of said luminous beam, the second spark-gap means being formed by said second electrode and a third electrode, said third electrode being aligned with said rst and second electrodes.

10. The system as claimed in claim 9 further including means for focusing said luminous beam on a surface of said screen, said focusing means being positioned in the bore of said second electrode.

References Cited UNITED STATES PATENTS 3,059,149 10/1962 Salisbury 315-267 3,405,316 10/1968 Osial et al. 315-243 JOHN W. HUCKERT, Primary Examiner SIMON BRODER, Assistant Examiner U.S. C1. X.R. 

